ci: reduce number of parallel build jobs on GitHub Actions to two

Linux and Windows machines of the GitHub-hosted runners only have
two cores, so running four parallel jobs is not really providing
any benefit to reduce the build time.

Author: striezel

.github/workflows/clang.yml

@@ -26,7 +26,7 @@ jobs:
           mkdir build
           cd build
           cmake ../
-          make -j4
+          make -j2
       - name: Run tests
         run: |
           cd "$GITHUB_WORKSPACE/build"

.github/workflows/gcc.yml

@@ -26,7 +26,7 @@ jobs:
           mkdir build
           cd build
           cmake ../
-          make -j4
+          make -j2
       - name: Run tests
         run: |
           cd "$GITHUB_WORKSPACE/build"